Throughout the specification, the terms "forward" and "rearward" are to be understood as referring to "in the direction to the front of the subject seat" and "in the direction to the rear of the subject seat", respectively and "upward" and "downward" to be taken as upward and downward with respect to the subject seat, respectively.
For increased riding comfort and for minimum occupant fatigue, a vehicle seat backrest must support the lumbar area of the seat occupant's back. But, it is recognized that the construction of universally satisfactory permanent seat structures can not be effected because of difference in individual body sizes and proportions.
In view of the above, it has become popular to equip a vehicle seat with a so-called "adjustable lumbar supporter" held in the backrest of the seat. However, such an adjustable lumbar supporter conventionally used still has a limitation in setting a plurality of positions. In fact, the adjustment of the supporter is limited to either forward-rearward positioning or upward-downward positioning. More specifically, the positioning of the lumbar supporter is made by moving it monodirectionally. Thus, the most desired and effective positioning of the supporter for properly supporting the lumbar area of the seated occupant has not been obtained.